It's gonna happen. Yuki is not going to suddenly side with the empire, so her switching sides is out of the question.
Stella has very little that binds her emotionally to the empire, but she does have her lost mother on the Kingdom's side.
The synopsis and the OP make it obvious that both of them are going to interact in some way, and the OP sure doesn't look like it will be a hostile confrontation.
Aine will not accept anything but total surrender, I highly doubt that Stella could convince him otherwise since she's a nobody when it comes to rank.
So take a guess who is going to switch sides.